1. Field of the Invention
The present invention relates to a pneumatic tire and, more specifically, to a run-flat tire that enables safe drive over a long distance even with a decreased inner pressure at the time of puncture.
2. Description of the Background Art
Generally, at the time of a tire puncture, a side-wall portion 31 having low rigidity of the tire is folded and protruded in the widthwise direction of the tire as shown in FIG. 6, and a tread portion 32 moves inward in the radial direction of the tire, so that the tire becomes flat. When running continues, a tire bead portion 33 falls into a rim well 34, possibly causing a hazardous state in which the tire drops off from the rim, disabling steering, or an inner upper end 33a of the bead portion is repeatedly subjected to severe friction with the tire inner region 32a of the tread portion, damaging the tire.
Conventionally, as a structure of a so-called run-flat tire enabling safe drive even at the time of tire puncture, the following structures have been proposed: a structure in which a crescent shape reinforcing rubber layer is arranged in contact with an inner side of a tire carcass over the bead portion to a shoulder portion, with its thickness gradually reduced toward opposing ends, or a structure in which a reinforcing rubber is arranged from the bead portion to an end of the thread portion between the carcass body and a folded portion thereof (Japanese Patent Laying-Open No. 10-244817); and a structure in which two-layered reinforcing rubber is arranged between a plurality of carcass plies or reinforcing plies (U.S. Pat. No. 5,427,166).
The above described prior art contemplates to reduce tire deformation at the time of puncture to avoid the hazardous state described above, by increasing rigidity of the side-wall portion by the reinforcing rubber. However, the reinforcing rubber layer arranged on an inner peripheral side of the carcass body portion as described above increases local strain at the side-wall portion at the time of tire puncture, and therefore damage to the side-wall portion of the tire cannot be alleviated. In order to further enhance the effect of reinforcement, the thickness of the reinforcing rubber layer must naturally be increased. This, however, increases the weight of the tire, and decreases fuel efficiency of the vehicle.